Bulk monocrystalline gallium nitride

ABSTRACT

The present invention refers to an ammonobasic method for preparing a gallium-containing nitride crystal, in which gallium-containing feedstock is crystallized on at least one crystallization seed in the presence of an alkali metal-containing component in a supercritical nitrogen-containing solvent. The method can provide monocrystalline gallium-containing nitride crystals having a very high quality.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention refers to processes for obtaining agallium-containing nitride crystal by an ammonobasic method as well asthe gallium-containing nitride crystal itself. Furthermore, an apparatusfor conducting the various methods is disclosed.

[0003] 2. Discussion of the Related Art

[0004] Optoelectronic devices based on nitrides are usually manufacturedon sapphire or silicon carbide substrates that differ from the depositednitride layers (so-called heteroepitaxy). In the most often usedMetallo-Organic Chemical Vapor Deposition (MOCVD) method, the depositionof GaN is performed from ammonia and organometallic compounds in the gasphase, and the growth rates achieved make it impossible to provide abulk layer. The application of a buffer layer reduces the dislocationdensity, but not more than to approx. 10⁸/cm². Another method has alsobeen proposed for obtaining bulk monocrystalline gallium nitride. Thismethod consists of an epitaxial deposition employing halides in a vaporphase and is called Halide vapor Phase Epitaxy (HVPE) [“Opticalpatterning of GaN films” M. K. Kelly, 0. Ambacher, Appl. Phys. Lett. 69(12) (1996) and “Fabrication of thin-film InGaN light-emitting diodemembranes” W. S. Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)].This method allows for the preparation of GaN substrates having a 2-inchdiameter.

[0005] However, their quality is not sufficient for laser diodes,because the dislocation density continues to be approx. 10⁷ to approx.10⁹/cm². Recently, the method of Epitaxial Lateral Overgrowth (ELOG) hasbeen used for reducing the dislocation density. In this method the GaNlayer is first grown on a sapphire substrate and then a layer with SiO₂is deposited on it in the form of strips or a lattice. On the thusprepared substrate, in turn, the lateral growth of GaN may be carriedout leading to a dislocation density of approx. 10⁷/cm².

[0006] The growth of bulk crystals of gallium nitride and other metalsof group XIII (IUPAC, 1989) is extremely difficult. Standard methods ofcrystallization from melt and sublimation methods are not applicablebecause of the decomposition of the nitrides into metals and N₂. In theHigh Nitrogen Pressure (HNP) method [“Prospects for high-pressurecrystal growth of III-V nitrides” S. Porowski et al., Inst. Phys. Conf.Series, 137, 369 (1998)] this decomposition is inhibited by the use ofnitrogen under the high pressure. The growth of crystals is carried outin molten gallium, i.e. in the liquid phase, resulting in the productionof GaN platelets about 10 mm in size. Sufficient solubility of nitrogenin gallium requires temperatures of about 1500° C. and nitrogenpressures in the order of 15 kbar.

[0007] The use of supercritical ammonia has been proposed to lower thetemperature and decrease the pressure during the growth process ofnitrides. Peters has described the ammonothermal synthesis of aluminiumnitride [J. Cryst. Growth 104, 411-418 (1990)]. R. Dwilinski et al. haveshown, in particular, that it is possible to obtain a fine-crystallinegallium nitride by a synthesis from gallium and ammonia, provided thatthe latter contains alkali metal amides (KNH₂ or LiNH₂) . The processeswere conducted at temperatures of up to 550° C. and under a pressure of5 kbar, yielding crystals about 5 μm in size [“AMMONO method of BN, AlN,and GaN synthesis and crystal growth”, Proc. EGW-3, Warsaw, Jun. 22-24,1998, MRS Internet Journal of Nitride Semiconductor Research,http://nsr.mij.mrs.org/3/25]. Another supercritical ammonia method,where a fine-crystalline GaN is used as a feedstock together with amineralizer consisting of an amide (KNH₂) and a halide (KI) alsoprovided for recrystallization of gallium nitride [“Crystal growth ofgallium nitride in supercritical ammonia” J. W. Kolis et al., J. Cryst.Growth 222, 431-434 (2001)]. The recrystallization process conducted at400° C. and 3.4 kbar resulted in GaN crystals about 0.5 mm in size. Asimilar method has also been described in Mat. Res. Soc. Symp. Proc.Vol. 495, 367-372 (1998) by J. W. Kolis et al. However, using thesesupercritical ammonia processes, no production of bulk monocrystallinewas achieved because no chemical transport processes were observed inthe supercritical solution, in particular no growth on seeds wasconducted.

SUMMARY OF THE INVENTION

[0008] Therefore, it is an object of the present invention to provide animproved method of preparing a gallium-containing nitride crystal.

[0009] The lifetime of optical semiconductor devices depends primarilyon the crystalline quality of the optically active layers, andespecially on the surface dislocation density. In case of GaN basedlaser diodes, it is beneficial to lower the dislocation density in theGaN substrate layer to less than 10⁶/cm², and this has been extremelydifficult to achieve using the methods known so far. Therefore, afurther object of the invention is to provide gallium-containing nitridecrystals having a quality suitable for use as substrates foroptoelectronics.

[0010] The above objects are achieved by the subject matter recited inthe appended claims. In particular, in one embodiment the presentinvention refers to a process for obtaining a gallium-containing nitridecrystal, comprising the steps of:

[0011] providing a gallium-containing feedstock, an alkalimetal-containing component, at least one crystallization seed and anitrogen-containing solvent in at least one container;

[0012] bringing the nitrogen-containing solvent into a supercriticalstate;

[0013] (iii) at least partially dissolving the gallium-containingfeedstock at a first temperature and at a first pressure; and

[0014] (iv) crystallizing gallium-containing nitride on thecrystallization seed at a second temperature and at a second pressurewhile the nitrogen-containing solvent is in the supercritical state;

[0015] wherein at least one of the following criteria is fulfilled:

[0016] (a) the second temperature is higher than the first temperature;and

[0017] (b) the second pressure is lower than the first pressure.

[0018] In a second embodiment a process for preparing agallium-containing nitride crystal is described which comprises thesteps of:

[0019] (i) providing a gallium-containing feedstock comprising at leasttwo different components, an alkali metal-containing component, at leastone crystallization seed and a nitrogen-containing solvent in acontainer having a dissolution zone and a crystallization zone, wherebythe gallium-containing feedstock is provided in the dissolution zone andthe at least one crystallization seed is provided in the crystallizationzone;

[0020] (ii) subsequently bringing the nitrogen-containing solvent into asupercritical state;

[0021] (iii) subsequently partially dissolving the gallium-containingfeedstock at a dissolution temperature and at a dissolution pressure inthe dissolution zone, whereby a first component of thegallium-containing feedstock is substantially completely dissolved and asecond component of the gallium-containing feedstock as well as thecrystallization seed remain substantially undissolved so that anundersaturated solution with respect to gallium-containing nitride isobtained;

[0022] (iv) subsequently setting the conditions in the crystallizationzone at a second temperature and at a second pressure so thatover-saturation with respect to gallium-containing nitride is obtainedand crystallization of gallium-containing nitride occurs on the at leastone crystallization seed and simultaneously setting the conditions inthe dissolution zone at a first temperature and at a first pressure sothat the second component of the gallium-containing feedstock isdissolved;

[0023] wherein the second temperature is higher than the firsttemperature.

[0024] A gallium-containing nitride crystal obtainable by one of theseprocesses is also described. Further subject matter of the invention area gallium-containing nitride crystal having a surface area of more than2 cm² and having a dislocation density of less than 10⁶/cm² and agallium-containing nitride crystal having a thickness of at least 200 μmand a full width at half maximum (FWHM) of X-ray rocking curve from(0002) plane of 50 arcsec or less.

[0025] The invention also provides an apparatus for obtaining agallium-containing nitride crystal comprising an autoclave (1) having aninternal space and comprising at least one device (4, 5, 6) for heatingthe autoclave to at least two zones having different temperatures,wherein the autoclave comprises a device which separates the internalspace into a dissolution zone (13) and a crystallization zone (14).

[0026] In a yet another embodiment, a process for preparing a bulkmonocrystalline gallium-containing nitride in an autoclave is disclosed,which comprises the steps of providing a supercritical ammonia solutioncontaining gallium-containing nitride with ions of alkali metals, andrecrystallizing said gallium-containing nitride selectively on acrystallization seed from said supercritical ammonia solution by meansof the negative temperature coefficient of solubility and/or by means ofthe positive pressure coefficient of solubility.

[0027] A process for controlling recrystallization of agallium-containing nitride in a supercritical ammonia solution whichcomprises steps of providing a supercritical ammonia solution containinga gallium-containing nitride as a gallium complex with ions of alkalimetal and NH₃ solvent in an autoclave and decreasing the solubility ofsaid gallium-containing nitride in the supercritical ammonia solution ata temperature less than that of dissolving gallium-containing nitridecrystal and/or at a pressure higher than that of dissolvinggallium-containing nitride crystal is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 shows the dependency of the solubility ofgallium-containing nitride in supercritical ammonia that containspotassium amide (with KNH₂:NH₃=0.07) on pressure at T=400° C. and T=500°C.

[0029]FIG. 2 shows the diagram of time variations of temperature in anautoclave at constant pressure for Example 1.

[0030]FIG. 3 shows the diagram of time variations of pressure in anautoclave at constant temperature for Example 2.

[0031]FIG. 4 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 3.

[0032]FIG. 5 shows the diagram of time variations of temperature in anautoclave for Example 4.

[0033]FIG. 6 shows the diagram of time variations of temperature in anautoclave for Example 5.

[0034]FIG. 7 shows the diagram of time variations of temperature in anautoclave for Example 6.

[0035]FIG. 8 shows the diagram of time variations of temperature in anautoclave for Example 7.

[0036]FIG. 9 shows a schematic axial cross section of an autoclave asemployed in many of the examples, mounted in the furnace.

[0037]FIG. 10 is a schematic perspective drawing of an apparatusaccording to the present invention.

[0038]FIG. 11 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 8.

[0039]FIG. 12 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 9.

[0040]FIG. 13 shows the diagram of time variations of temperature in anautoclave at constant volume for Example 10.

[0041]FIG. 14 shows the diagram of time variations of temperature in anautoclave at constant volume for Examples 11 and 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] In the present invention the following definitions apply.

[0043] Gallium-containing nitride means a nitride of gallium andoptionally other element(s) of group XIII (according to IUPAC, 1989). Itincludes, but is not restricted to, the binary compound GaN, ternarycompounds such as AlGaN, InGaN and also AlInGaN, where the ratio of theother elements of group XIII to Ga can vary in a wide range.

[0044] Bulk monocrystalline gallium-containing nitride means amonocrystalline substrate made of gallium-containing nitride from whichoptoelectronic devices such as LED or LD can be formed by epitaxialmethods such as MOCVD and HVPE.

[0045] Supercritical solvent means a fluid in a supercritical state. Itcan also contain other components in addition to the solvent itself aslong as these components do not substantially influence or disturbfunction of supercritical solvent. In particular, the solvent cancontain ions of alkali metals.

[0046] Supercritical solution is used when referring to thesupercritical solvent when it contains gallium in a soluble formoriginating from the dissolution of gallium-containing feedstock.

[0047] Dissolution of gallium-containing feedstock means a process(either reversible or irreversible) in which said feedstock is taken upto the supercritical solvent as gallium in a soluble form, possiblygallium-complex compounds.

[0048] Gallium-complex compounds are complex compounds, in which agallium atom is a coordination center surrounded by ligands, such as NH₃molecules or its derivatives, like NH₂ ⁻, NH²⁻, etc.

[0049] Negative temperature coefficient of solubility means that thesolubility of the respective compound is a monotonically decreasingfunction of temperature if all other parameters are kept constant.Similarly, positive pressure coefficient of solubility means that, ifall other parameters are kept constant, the solubility is amonotonically increasing function of pressure. In our research we showedthat the solubility of gallium-containing nitride in supercriticalnitrogen-containing solvents, such as ammonia, possesses a negativetemperature coefficient and a positive pressure coefficient intemperatures ranging at least from 300 to 600° C. and pressures from 1to 5.5 kbar.

[0050] Over-saturation of supercritical solution with respect togallium-containing nitride means that the concentration of gallium in asoluble form in said solution is higher than that in equilibrium (i.e.it is higher than solubility). In the case of dissolution ofgallium-containing nitride in a closed system, such an over-saturationcan be achieved by either increasing the temperature and/or decreasingthe pressure.

[0051] Spontaneous crystallization means an undesired process wherenucleation and growth of the gallium-containing nitride fromover-saturated supercritical solution take place at any site within anautoclave except at the surface of a seed crystal where the growth isdesired. Spontaneous crystallization also comprises nucleation anddisoriented growth on the surface of seed crystal.

[0052] Selective crystallization on a seed means a process ofcrystallization on a seed carried out without spontaneouscrystallization.

[0053] Autoclave means a closed container which has a reaction chamberwhere the ammonobasic process according to the present invention iscarried out.

[0054] The present invention can provide a gallium-containing nitridemonocrystal having a large size and a high quality. Suchgallium-containing nitride crystals can have a surface area of more than2 cm² and a dislocation density of less than 10⁶/cm². Gallium-containingnitride crystals having a thickness of at least 200 μm (preferably atleast 500 μm) and a FWHM of 50 arcsec or less can also be obtained.Depending on the crystallization conditions, it possible to obtaingallium-containing nitride crystals having a volume of more than 0.05cm³, preferably more than 0.1 cm³ using the processes of the invention.

[0055] As was explained above, the gallium-containing nitride crystal isa crystal of nitride of gallium and optionally other element(s) of GroupXIII (the numbering of the Groups is given according to the IUPACconvention of 1989 throughout this application). These compounds can berepresented by the formula Al_(x)Ga_(1−x−y)In_(y)N, wherein 0≦x<1,0≦y<1, 0≦x+y<1; preferably 0x<0.5 and 0y<0.5. Although in a preferredembodiment, the gallium-containing nitride is gallium nitride, in afurther preferred embodiment part (e.g. up to 50 mol.-%) of the galliumatoms can be replaced by one or more other elements of Group XIII(especially Al and/or In).

[0056] The gallium-containing nitride may additionally include at leastone donor and/or at least one acceptor and/or at least one magneticdopant to alter the optical, electrical and magnetic properties of thesubstrate. Donor dopants, acceptor dopants and magnetic dopants arewell-known in the art and can be selected according to the desiredproperties of the substrate. Preferably the donor dopants are selectedfrom the group consisting of Si and O. As acceptor donors Mg and Zn arepreferred. Any known magnetic dopant can be included into the substratesof the present invention. A preferred magnetic dopant is Mn and possiblyalso Ni and Cr. The concentrations of the dopants are well-known in theart and depend on the desired end application of the nitride. Typicallythe concentrations of these dopants are ranging from 10¹⁷ to 10²¹/cm³.

[0057] Due to the production process the gallium-containing nitridecrystal can also contain alkali elements, usually in an amount of morethan about 0.1 ppm. Generally it is desired to keep the alkali elementscontent lower than 10 ppm, although it is difficult to specify whatconcentration of alkali metals in gallium-containing nitride has adisadvantageous influence on its properties.

[0058] It is also possible that halogens are present in thegallium-containing nitride. The halogens can be introduced eitherintentionally (as a component of the mineralizer) or unintentionally(from impurities of the mineralizer or the feedstock). It is usuallydesired to keep the halogen content of the gallium-containing nitridecrystal in the range of about 0.1 ppm or less.

[0059] The process of the invention is a supercritical crystallizationprocess, which includes at least two steps: a dissolution step at afirst temperature and at a first pressure and a crystallization step ata second temperature and at a second pressure. Since generally highpressures and/or high temperatures are involved, the process accordingto the invention is preferably conducted in an autoclave. The two steps(i.e. the dissolution step and the crystallization step) can either beconducted separately or can be conducted at least partiallysimultaneously in the same reactor.

[0060] For conducting the two steps separately the process can beconducted in one reactor but the dissolution step is conducted beforethe crystallization step. In this embodiment the reactor can have theconventional construction of one single chamber. The process of theinvention in the two-step embodiment can be conducted using constantpressure and two different temperatures or using constant temperatureand two different pressures. It is also possible to use two differentpressures and two different temperatures. The exact values of pressureand temperature should be selected depending on the feedstock, thespecific nitride to be prepared and the solvent. Generally the pressureis in the range of 1 to 10 kbar, preferably 1 to 5.5 and more preferably1.5 to 3 kbar. The temperature is in the range of 100° C. to 800° C.,preferably 300° C. to 600° C., more preferably 400° C. to 550° C. If twodifferent pressures are employed, the difference in pressure should befrom 0.1 kbar to 9 kbar, preferably from 0.2 kbar to 3 kbar. However, ifthe dissolution and crystallization are controlled by the temperature,the difference in temperature should be at least 1° C., and preferablyfrom 5° C. to 150° C.

[0061] In a preferred embodiment, the dissolution step and thecrystallization step are conducted at least partially simultaneously inthe same container. For such an embodiment the pressure is practicallyuniform within the container, while the temperature difference betweenthe dissolution zone and crystallization zone should be at least 1° C.,and preferably is from 5° C. to 150° C. Furthermore, the temperaturedifference between the dissolution zone and crystallization zone shouldbe controlled so as to ensure chemical transport in the supercriticalsolution, which takes place through convection.

[0062] A possible construction of a preferred container is given in FIG.9. For conciseness and ease of understanding in the following, theprocess will be explained particularly with respect to this preferredembodiment. However, the invention can be conducted with differentcontainer constructions as long as the principles outlined in thespecification and the claims are adhered to.

[0063] In a preferred embodiment of the invention, the process can beconducted in an apparatus comprising an autoclave 1 having an internalspace and comprising at least one device 4, 5, 6 for heating theautoclave to at least two zones having different temperatures, whereinthe autoclave comprises a device which separates the internal space intoa dissolution zone 13 and a crystallization zone 14. These two zoneshaving different temperatures should preferably coincide with thedissolution zone 13 and the crystallization zone 14. The device whichseparates the internal space of the autoclave can be, for example, atleast one baffle 12 having at least one opening 2. Examples are baffleshaving a central opening, circumferential openings or a combinationthereof. The size of the opening(s) 2 should be large enough to allowtransport between the zones but should be sufficiently small to maintaina temperature gradient in the reactor. The appropriate size of theopening depends on the size and the construction of the reactor and canbe easily determined by a person skilled in the art.

[0064] In one embodiment, two different heating devices can be employed,the position of which corresponds to dissolution zone 13 and thecrystallization zone 14. However, it has been observed that transport ofgallium in a soluble form from the dissolution zone 13 to thecrystallization zone 14 can be further improved if a third cooling means6 is present between the first and the second heating devices and islocated at approximately the position of the separating device. Thecooling means 6 can be realized by liquid (e.g. water) cooling orpreferably by fan cooling. The heating devices are powered electrically,by either inductive or, preferably, resistive heating means. Use of aheating 4—cooling 6—heating 5 configuration gives wider possibilities informing the desired temperature distribution within the autoclave. Forexample, it enables to obtain low temperature gradients in most of thecrystallization zone 14 and of the dissolution zone 13, and a hightemperature gradient in the region of baffle 12.

[0065] When the process of the present invention is conducted, providinga gallium-containing feedstock, an alkali metal-containing component, atleast one crystallization seed and a nitrogen-containing solvent areprovided in at least one container. In the preferred apparatus describedabove, the gallium-containing feedstock is placed in the dissolutionzone and the at least one crystallization seed is placed in thecrystallization zone. The alkali metal containing component is alsopreferably placed in the dissolution zone. Then the nitrogen-containingsolvent is added into the container, which is then closed. Subsequentlythe nitrogen-containing solvent is brought into a supercritical state,e.g. by pressure and/or heat.

[0066] In the present invention any materials containing gallium, whichare soluble in the supercritical solvent under the conditions of thepresent invention, can be used as a gallium-containing feedstock.Typically the gallium-containing feedstock will be a substance ormixture of substances, which contains at least gallium, and optionallyalkali metals, other Group XIII elements, nitrogen, and/or hydrogen,such as metallic Ga, alloys and inter-metallic compounds, hydrides,amides, imides, amido-imides, azides. Suitable gallium-containingfeedstocks can be selected from the group consisting of gallium nitrideGaN, azides such as Ga(N₃)₃, imides such as Ga₂(NH)₃, amido-imides suchas Ga(NH)NH₂, amides such as Ga(NH₂)₃, hydrides such as GaH₃,gallium-containing alloys, metallic gallium and mixtures thereof.Preferred feedstocks are metallic gallium and gallium nitride andmixtures thereof. Most preferably, the feedstock is metallic gallium orgallium nitride. If other elements of Group XIII are to be incorporatedinto the gallium-containing nitride crystal, corresponding compounds ormixed compounds including Ga and the other Group XIII element can beused. If the substrate is to contain dopants or other additives,precursors thereof can be added to the feedstock.

[0067] The form of the feedstock is not particularly limited and it canbe in the form of one or more pieces or in the form of a powder. If thefeedstock is in the form of a powder, care should be taken thatindividual powder particles are not transported from the dissolutionzone to the crystallization zone, where they can cause irregularcrystallization. It is preferable that the feedstock is in one or morepieces and that the surface area of the feedstock is larger than that ofthe crystallization seed.

[0068] The nitrogen-containing solvent employed in the present inventionmust be able to form a supercritical fluid, in which gallium can bedissolved in the presence of alkali metal ions. Preferably the solventis ammonia, a derivative thereof or mixtures thereof. An example of asuitable ammonia derivative is hydrazine. Most preferably the solvent isammonia. To reduce corrosion of the reactor and to avoid side-reactions,halogens e.g. in the form of halides are preferably not intentionallyadded into the reactor. Although traces of halogens may be introducedinto the system in the form of unavoidable impurities of the startingmaterials, care should be taken to keep the amount of halogen as low aspossible. Due to the use of a nitrogen-containing solvent such asammonia it is not necessary to include nitride compounds into thefeedstock. Metallic gallium (or aluminum or indium) can be employed asthe source material while the solvent provides the nitrogen required forthe nitride formation.

[0069] It has been observed that the solubility of gallium-containingfeedstock, such as gallium and corresponding elements of Group XIIIand/or their compounds, can be significantly improved by the presence ofat least one type of alkali metal-containing component as asolubilization aid (“mineralizer”). Lithium, sodium and potassium arepreferred as alkali metals, wherein sodium and potassium are morepreferred. The mineralizer can be added to the supercritical solvent inelemental form or preferably in the form of its compound. Generally thechoice of the mineralizer depends on the solvent employed in theprocess. According to our investigations, alkali metal having a smallerion radius can provide lower solubility of gallium-containing nitride inthe supercritical ammonia solvent than that obtained with alkali metalshaving a larger ion radius. For example, if the mineralizer is in theform of a compound, it is preferably an alkali metal hydride such as MH,an alkali metal nitride such as M₃N, an alkali metal amide such as MNH₂,an alkali metal imide such as M₂NH or an alkali metal azide such as MN₃(wherein M is an alkali metal). The concentration of the mineralizer isnot particularly restricted and is selected so as to ensure adequatelevels of solubility of both feedstock (the starting material) andgallium-containing nitride (the resulting product). It is usually in therange of 1:200 to 1:2, in the terms of the mols of the metal ion basedon the mols of the solvent (molar ratio). In a preferred embodiment theconcentration is from 1:100 to 1:5, more preferably 1:20 to 1:8 mols ofthe metal ion based on the mols of the solvent.

[0070] The presence of the alkali metal in the process can lead toalkali metal elements in the thus prepared substrates. It is possiblethat the amount of alkali metal elements is more than about 0.1 ppm,even more than 10 ppm. However, in these amounts the alkali metals donot detrimentally effect the properties of the substrates. It has beenfound that even at an alkali metal content of 500 ppm, the operationalparameters of the substrate according to the invention are stillsatisfactory.

[0071] The dissolved feedstock crystallizes in the crystallization stepunder the low solubility conditions on the crystallization seed(s) whichare provided in the container. The process of the invention allows bulkgrowth of monocrystalline gallium-containing nitride on thecrystallization seed(s) and in particular leads to the formation ofstoichiometric nitride in the form of a monocrystalline bulk layer onthe crystallization seed(s).

[0072] Various crystals can be used as crystallization seeds in thepresent invention, however, it is preferred that the chemical andcrystallographic constitution of the crystallization seeds is similar tothose of the desired layer of bulk monocrystalline gallium-containingnitride. Therefore, the crystallization seed preferably comprises acrystalline layer of gallium-containing nitride and optionally one ormore other elements of Group XIII. To facilitate crystallization of thedissolved feedstock, the defects surface density of the crystallizationseed is preferably less than 10⁶/cm². Suitable crystallization seedsgenerally have a surface area of 8×8 mm² or more and thickness of 100 μmor more, and can be obtained e.g. by HVPE.

[0073] After the starting materials have been introduced into thecontainer and the nitrogen-containing solvent has been brought into itssupercritical state, the gallium-containing feedstock is at leastpartially dissolved at a first temperature and a first pressure, e.g. inthe dissolution zone of an autoclave. Gallium-containing nitridecrystallizes on the crystallization seed (e.g. in the crystallizationzone of an autoclave) at a second temperature and at a second pressurewhile the nitrogen-containing solvent is in the supercritical state,wherein the second temperature is higher than the first temperatureand/or the second pressure is lower than the first pressure. If thedissolution and the crystallization steps take place simultaneously inthe same container, the second pressure is essentially equal to thefirst pressure.

[0074] This is possible since the solubility of gallium-containingnitride under the conditions of the present invention shows a negativetemperature coefficient and a positive pressure coefficient in thepresence of alkali metal ions. Without wishing to be bound by theory, itis postulated that the following processes occur. In the dissolutionzone, the temperature and pressure are selected such that thegallium-containing feedstock is dissolved and the nitrogen-containingsolution is undersaturated with respect to gallium-containing nitride.At the crystallization zone, the temperature and pressure are selectedsuch that the solution, although it contains approximately the sameconcentration of gallium as in the dissolution zone, is over-saturatedwith respect to gallium-containing nitride. Therefore, crystallizationof gallium-containing nitride on the crystallization seed occurs. Thisis illustrated in FIG. 15. Due to the temperature gradient, pressuregradient, concentration gradient, different chemical or physicalcharacter of dissolved feedstock and crystallized product etc., galliumis transported in a soluble form from the dissolution zone to thecrystallization zone. In the present invention this is referred to aschemical transport of gallium-containing nitride in the supercriticalsolution. It is postulated that the soluble form of gallium is a galliumcomplex compound, with Ga atom in the coordination center surrounded byligands, such as NH₃ molecules or its derivatives, like NH₂ ⁻, NH²⁻,etc.

[0075] This theory is equally applicable for all gallium-containingnitrides, such as AlGaN, InGaN and AlInGaN as well as GaN (the mentionedformulas are only intended to give the components of the nitrides. It isnot intended to indicate their relative amounts). In such cases alsoaluminum and/or indium in a soluble form have to be present in thesupercritical solution.

[0076] In a preferred embodiment of the invention, thegallium-containing feedstock is dissolved in at least two steps. In thisembodiment, the gallium-containing feedstock generally comprises twokinds of starting materials which differ in solubility. The differencein solubility can be achieved chemically (e.g. by selecting twodifferent chemical compounds) or physically (e.g. by selecting two formsof the same compound having definitely different surface areas, likemicrocrystalline powder and big crystals). In a preferred embodiment,the gallium-containing feedstock comprises two different chemicalcompounds such as metallic gallium and gallium nitride which dissolve atdifferent rates. In a first dissolution step, the first component of thegallium-containing feedstock is substantially completely dissolved at adissolution temperature and at a dissolution pressure in the dissolutionzone. The dissolution temperature and the dissolution pressure, whichcan be set only in the dissolution zone or preferably in the wholecontainer, are selected so that the second component of thegallium-containing feedstock and the crystallization seed(s) remainsubstantially undissolved. This first dissolution step results in anundersaturated or at most saturated solution with respect togallium-containing nitride. For example, the dissolution temperature canbe 100° C. to 350° C., preferably from 150° C. to 300° C. Thedissolution pressure can be 0.1 kbar to 5 kbar, preferably from 0.1 kbarto 3 kbar. Generally the dissolution temperature is lower than the firsttemperature.

[0077] Subsequently the conditions in the crystallization zone are setat a second temperature and at a second pressure so that over-saturationwith respect to gallium-containing nitride is obtained andcrystallization of gallium-containing nitride occurs on the at least onecrystallization seed. Simultaneously the conditions in the dissolutionzone are set at a first temperature and at a first pressure (practicallyequal to the second pressure) so that the second component of thegallium-containing feedstock is now dissolved (second dissolution step).As explained above the second temperature is higher than the firsttemperature and/or the second pressure is lower than the first pressureso that the crystallization can take advantage of the negativetemperature coefficient of solubility and/or by means of the positivepressure coefficient of solubility. Preferably the first temperature isalso higher than the dissolution temperature. During the seconddissolution step and the crystallization step, the system should be in astationary state so that the concentration of gallium in thesupercritical solution remains substantially constant, i.e. the sameamount of gallium should be dissolved per unit of time as iscrystallized in the same unit of time. This allows for the growth ofgallium-containing nitride crystals of especially high quality and largesize.

[0078] Typical pressures for the crystallization step and the seconddissolution step are in the range of 1 to 10 kbar, preferably 1 to 5.5and more preferably 1.5 to 3 kbar. The temperature is in the range of100 to 800° C., preferably 300 to 600° C., more preferably 400 to 550°C. The difference in temperature should be at least 1° C., andpreferably from 5° C. to 150° C. As explained above, the temperaturedifference between the dissolution zone and crystallization zone shouldbe controlled so as to ensure a chemical transport in the supercriticalsolution, which takes place through convection in an autoclave.

[0079] In the process of the invention, the crystallization should takeplace selectively on the crystallization seed and not on a wall of thecontainer. Therefore, the over-saturation extent with respect to thegallium-containing nitride in the supercritical solution in thecrystallization zone should be controlled so as to be below thespontaneous crystallization level where crystallization takes place on awall of the autoclave as well as on the seed, i.e. the level at whichspontaneous crystallization occurs. This can be achieved by adjustingthe chemical transport rate and the crystallization temperature and/orpressure. The chemical transport is related on the speed of a convectionflow from the dissolution zone to the crystallization zone, which can becontrolled by the temperature difference between the dissolution zoneand the crystallization zone, the size of the opening(s) of baffle(s)between the dissolution zone and the crystallization zone and so on.

[0080] The performed tests showed that the best bulk monocrystallinegallium nitride obtained had the dislocation density close to 10⁴/cm²with simultaneous FWHM of X-ray rocking curve from (0002) plane below 60arcsec, providing an appropriate quality and durability for opticsemiconductor devices produced with its use. The gallium-containingnitride of the present invention typically has a wurzite structure.

[0081] Feedstock material can also be prepared using a method similar tothose described above. The method involves the steps of:

[0082] (i) providing a gallium-containing feedstock, an alkalimetal-containing component, at least one crystallization seed and anitrogen-containing solvent in a container having at least one zone;

[0083] (ii) subsequently bringing the nitrogen-containing solvent into asupercritical state;

[0084] (iii) subsequently dissolving the gallium-containing feedstock(such as metallic gallium or aluminium or indium, preferably metallicgallium) at a dissolution temperature and at a dissolution pressure,whereby the gallium-containing feedstock is substantially completelydissolved and the crystallization seed remains substantially undissolvedso that an undersaturated solution with respect to gallium-containingnitride is obtained;

[0085] (iv) subsequently setting the conditions in the container at asecond temperature and at a second pressure so that over-saturation withrespect to gallium-containing nitride is obtained and crystallization ofgallium-containing nitride occurs on the at least one crystallizationseed;

[0086] wherein the second temperature is higher than the dissolutiontemperature.

[0087] The conditions described above with respect to the dissolutiontemperature and the second temperature also apply in this embodiment.

[0088] Gallium-containing nitride exhibits good solubility insupercritical nitrogen-containing solvents (e.g. ammonia), providedalkali metals or their compounds, such as KNH₂, are introduced into it.FIG. 1 shows the solubility of gallium-containing nitride in asupercritical solvent versus pressure for temperatures of 400 and 500°C. wherein the solubility is defined by the molar percentage:S_(m)≡GaN^(solvent):(KNH₂+NH₃) 100%. In the presented case the solventis the KNH₂ solution in supercritical ammonia of a molar ratiox≡KNH₂:NH₃ equal to 0.07. For this case S_(m) should be a smoothfunction of only three parameters: temperature, pressure, and molarratio of mineralizer (i.e. S_(m)=S_(m)(T, p, x)). Small changes of S_(m)can be expressed as:

ΔS _(m)≈(∂S _(m) /∂T)|_(p,x) ΔT+(∂S _(m) /∂p)|_(T,x) Δp+(∂S _(m)/∂x)|_(T,p) Δx,

[0089] where the partial differentials (e.g. (∂S_(m)/∂T)|_(p,x))determine the behavior of S_(m) with variation of its parameters (e.g.T). In this specification the partial differentials are called“coefficients” (e.g. (∂S_(m)/∂T)|_(p,x) is a “temperature coefficient ofsolubility”).

[0090] The diagram shown FIG. 1 illustrates that the solubilityincreases with pressure and decreases with temperature, which means thatit possesses a negative temperature coefficient and a positive pressurecoefficient. Such features allow obtaining a bulk monocrystallinegallium-containing nitride by dissolution in the higher solubilityconditions, and crystallization in the lower solubility conditions. Inparticular, the negative temperature coefficient means that, in thepresence of temperature gradient, the chemical transport of gallium in asoluble form can take place from the dissolution zone having a lowertemperature to the crystallization zone having a higher temperature.

[0091] The process according to invention allows the growth of bulkmonocrystalline gallium-containing nitride on the seed and leads inparticular to creation of stoichiometric gallium nitride, obtained inthe form of monocrystalline bulk layer grown on a gallium-nitride seed.Since such a monocrystal is obtained in a supercritical solution thatcontains ions of alkali metals, it contains also alkali metals in aquantity higher than 0.1 ppm. Because it is desired to maintain a purelybasic character of a supercritical solution, mainly in order to avoidcorrosion of the apparatus, halides are preferably not intentionallyintroduced into the solvent. The process of the invention can alsoprovide a bulk monocrystalline gallium nitride in which part of thegallium, e.g. from 0.05 to 0.5 may be substituted by Al and/or In.Moreover, the bulk monocrystalline gallium nitride may be doped withdonor and/or acceptor and/or magnetic dopants. These dopants can modifyoptical, electric and magnetic properties of a gallium-containingnitride. With respect to the other physical properties, the bulkmonocrystalline gallium nitride can have a dislocation density below10⁶/cm², preferably below 10⁵/cm², or most preferably below 10⁴/cm².Besides, the FWHM of the X-ray rocking curve from (0002) plane can bebelow 600 arcsec, preferably below 300 arcsec, and most preferably below60 arcsec. The best bulk monocrystalline gallium nitride obtained mayhave dislocation density lower than 10⁴/cm² and simultaneously a FWHM ofX-ray rocking curve from (0002) plane below 60 arcsec.

[0092] Due to the good crystalline quality the obtainedgallium-containing nitride crystals obtained in the present invention,they may be used as a substrate material for optoelectronicsemiconductor devices based on nitrides, in particular for laser diodes.

[0093] The following examples are intended to illustrate the inventionand should not be construed as being limiting.

EXAMPLES

[0094] Since it is not possible to readily measure the temperature in anautoclave while in use under supercritical conditions, the temperaturein the autoclave was estimated by the following method. The outside ofthe autoclave is equipped with thermocouples near the dissolution zoneand the crystallization zone. For the calibration, additionalthermocouples were introduced into the inside of the empty autoclave inthe dissolution zone and the crystallization zone. The empty autoclavewas then heated stepwise to various temperatures and the values of thetemperature of the thermocouples inside the autoclave and outside theautoclave were measured and tabulated. For example, if the temperatureof the crystallization zone is determined to be 500° C. and thetemperature of the dissolution zone is 400° C. inside the emptyautoclave when the temperature measured by the outside thermocouples are480° C. and 395° C., respectively. It is assumed that undersupercritical crystallization conditions the temperatures in thecrystallization/dissolution zones will also be 500/400° C. whentemperatures of 480/395° C. are measured by the outside thermocouples.In reality, the temperature difference between the two zones can belower due to effective heat transfer through the supercritical solution.

Example 1

[0095] Two crucibles were placed into a high-pressure autoclave having avolume of 10.9 cm³. The autoclave is manufactured according to a knowndesign [H. Jacobs, D. Schmidt, Current Topics in Materials Science, vol.8, ed. E. Kaldis (North-Holland, Amsterdam, 1981), 381]. One of thecrucibles contained 0.4 g of gallium nitride in the form of 0.1 mm thickplates produced by the HVPE method as feedstock, while other contained agallium nitride seed of a double thickness weighing 0.1 g. The seed wasalso obtained by the HVPE method. Further, 0.72 g of metallic potassiumof 4N purity was placed in the autoclave, the autoclave was filled with4.81 g of ammonia and then closed. The autoclave was put into a furnaceand heated to the temperature of 400° C. The pressure within theautoclave was 2 kbar. After 8 days the temperature was increased to 500°C., while the pressure was maintained at the 2 kbar level and theautoclave was maintained under these conditions for another 8 days (FIG.2). As a result of this process, in which the dissolution andcrystallization steps were separated in time, the feedstock wascompletely dissolved and the recrystallization of gallium nitride layertook place on the partially dissolved seed. The two-sidedmonocrystalline layers had a total thickness of about 0.4 mm.

Example 2

[0096] Two crucibles were put into the above-mentioned high-pressureautoclave having a volume of 10.9 cm³. One of the crucibles contained0.44 g of gallium nitride in the form of 0.1 mm thick plates produced bythe HVPE method as feedstock, and the other contained a gallium nitrideseed of a double thickness weighing 0.1 g, also obtained by the HVPEmethod. Further, 0.82 g of metallic potassium of 4N purity was placed inthe autoclave, the autoclave was filled with 5.43 g of ammonia and thenclosed. The autoclave was put into a furnace and heated to temperatureof 500° C. The pressure within the autoclave was 3.5 kbar. After 2 daysthe pressure was lowered to 2 kbar, while the temperature was maintainedat the 500° C. level and the autoclave was maintained under theseconditions for another 4 days (FIG. 3). As a result of this process, thefeedstock was completely dissolved and the recrystallization of galliumnitride took place on the partially dissolved seed. The two-sidedmonocrystalline layers had a total thickness of about 0.25 mm.

Example 3

[0097] Two crucibles were placed into the above-mentioned high-pressureautoclave having a volume of 10.9 cm³. One of the crucibles contained0.3 g of the feedstock in the form of metallic gallium of 6N purity andthe other contained a 0.1 g gallium nitride seed obtained by the HVPEmethod. Further, 0.6 g of metallic potassium of 4N purity was placed inthe autoclave; the autoclave was filled with 4 g of ammonia and thenclosed. The autoclave was put into a furnace and heated to temperatureof 200° C. After 2 days the temperature was increased to 500° C., whilethe pressure was maintained at the 2 kbar level and the autoclave wasmaintained in these conditions for further 4 days (FIG. 4). As a resultof this process, the feedstock was completely dissolved and thecrystallization of gallium nitride took place on the seed. The two-sidedmonocrystalline layers had a total thickness of about 0.3 mm.

Example 4

[0098] This is an example of process, in which the dissolution andcrystallization steps take place simultaneously (recrystallizationprocess). In this example and all the following an apparatus is usedwhich is schematically shown in FIG. 9. The basic unit of the apparatusis the autoclave 1, which in this Example has a volume of 35.6 cm³. Theautoclave 1 is equipped with an installation 2 for providing chemicaltransport of the solvent in a supercritical solution inside theautoclave 1. For this purpose, the autoclave 1 is put into a chamber 3of a set of two furnaces 4 provided with heating devices 5 and a coolingdevices 6. The autoclave 1 is secured in a desired position with respectto the furnaces 4 by means of a screw-type blocking device 7. Thefurnaces 4 are mounted on a bed 8 and are secured by means of steeltapes 9 wrapped around the furnaces 4 and the bed 8. The bed 8 togetherwith the set of furnaces 4 is rotationally mounted in base 10 and issecured in a desired angular position by means of a pin interlock 11. Inthe autoclave 1, placed in the set of furnaces 4, the convective flow ofsupercritical solution takes place as determined by the installation 2.The installation 2 is in the form of a horizontal baffle 12 having acentral opening. The baffle 12 separates the dissolution zone 13 fromthe crystallization zone 14 in the autoclave 1, and enables, togetherwith the adjustable tilting angle of the autoclave 1, controlling ofspeed and type of convective flow. The temperature level of theindividual zones in the autoclave 1 is controlled by means of a controlsystem 15 operating the furnaces 4. In the autoclave 1, the dissolutionzone 13 coincides with the low-temperature zone of the set of furnaces4, is located above the horizontal baffle 12 and the feedstock 16 is putinto this zone 13. On the other hand, the crystallization zone 14coincides with the high-temperature zone of the set of furnaces 4 and itis located below the horizontal baffle 12. The seed 17 is mounted inthis zone 14. The mounting location of the seed 17 is below theintersection of the rising and descending convective streams.

[0099] An amount of 3.0 g of gallium nitride produced by the HVPE methodwas placed in the high-pressure autoclave described above, which was setin the horizontal position. This gallium nitride had the form of platesof about 0.2 mm thickness, and it was distributed (roughly uniformly) inequal portions in the dissolution zone 13 and the crystallization zone14. The portion placed in the dissolution zone 13 played the role offeedstock, whereas the portion placed in the crystallization zone 14played the role of crystallization seeds. Metallic potassium of 4Npurity was also added in a quantity of 2.4 g. Then the autoclave 1 wasfilled with 15.9 g of ammonia (5N), closed, put into a set of furnaces 4and heated to a temperature of 450° C. The pressure inside the autoclave1 was approx. 2 kbar. During this stage, which lasted one day, a partialdissolution of gallium nitride was carried out in both zones. Then thetemperature of the crystallization zone 14 was increased to 500° C.while the temperature of the dissolution zone 13 was lowered to 400° C.and the autoclave 1 was kept in these conditions for 6 more days (FIG.5). As a final result of this process, partial dissolution of thefeedstock in the dissolution zone 13 and crystallization of galliumnitride on the gallium nitride seeds in the crystallization zone 14 tookplace.

Example 5

[0100] The above-mentioned high pressure autoclave 1 having a volume of35.6 cm³ was charged with feedstock in the form of a 3.0 g pellet ofsintered gallium nitride (introduced into the dissolution zone 13) twoseeds of gallium nitride obtained by the HVPE method and having the formof plates having a thickness of 0.4 mm and total weight of 0.1 g(introduced into the crystallization zone 14), as well as with 2.4 g ofmetallic potassium of 4N purity. Then the autoclave was filled with 15.9g of ammonia (5N) and closed. The autoclave 1 was then put into a set offurnaces 4 and heated to 450° C. The pressure inside the autoclave wasabout 2 kbar. After an entire day the temperature of the crystallizationzone 14 was raised to 480° C., while the temperature of dissolution zone13 was lowered to 420° C. and the autoclave was maintained under theseconditions for 6 more days (see FIG. 6). As a result of the process thefeedstock was partially dissolved in the dissolution zone 13 and galliumnitride crystallized on the seeds in the crystallization zone 14. Thetwo-sided monocrystalline layers had total thickness of about 0.2 mm.

Example 6

[0101] The above-mentioned high pressure autoclave 1 having a volume of35.6 cm³ (see FIG. 9) was charged with 1.6 g of feedstock in the form ofgallium nitride produced by the HVPE method and having the form ofplates having a thickness of about 0.2 mm (introduced into thedissolution zone 13), three gallium-nitride seeds of a thickness ofabout 0.35 mm and a total weight of 0.8 g, also obtained by the HVPEmethod (introduced into the crystallization zone 14), as well as with3.56 g of metallic potassium of 4N purity. The autoclave 1 was filledwith 14.5 g of ammonia (5N) and closed. Then the autoclave 1 was putinto a set of furnaces 4 and heated to 425° C. The pressure inside theautoclave was approx. 1.5 kbar. After an entire day the temperature ofdissolution zone 13 was lowered to 400° C. while the temperature ofcrystallization zone 14 was increased to 450° C. and the autoclave waskept in these conditions for 8 more days (see FIG. 7). After theprocess, the feedstock was found to be partially dissolved in thedissolution zone 13 and gallium nitride had crystallized on the seeds ofthe HVPE GaN in the crystallization zone 14. The two-sidedmonocrystalline layers had a total thickness of about 0.15 mm.

Example 7

[0102] The above-mentioned high pressure autoclave 1 having a volume of35.6 cm³ (see FIG. 9) was charged in its dissolution zone 13 with 2 g offeedstock in the form of gallium nitride produced by the HVPE method andhaving the form of plates having a thickness of about 0.2 mm, and 0.47 gof metallic potassium of 4N purity, and in its crystallization zone 14with three GaN seeds of a thickness of about 0.3 mm and a total weightof about 0.3 g also obtained by the HVPE method. The autoclave wasfilled with 16.5 g of ammonia (5N) and closed. Then the autoclave 1 wasput into a set of furnaces 4 and heated to 500° C. The pressure insidethe autoclave was approx. 3 kbar. After an entire day the temperature inthe dissolution zone 13 was reduced to 450° C. while the temperature inthe crystallization zone 14 was raised to 550° C. and the autoclave waskept under these conditions for the next 8 days (see FIG. 8). After theprocess, the feedstock was found to be partially dissolved in thedissolution zone 13 and gallium nitride had crystallized on seeds in thecrystallization zone 14. The two-sided monocrystalline layers had atotal thickness of about 0.4 mm.

Example 8

[0103] An amount of 1.0 g of gallium nitride produced by the HVPE methodwas put into the dissolution zone 13 of the high-pressure autoclave 1having a volume of 35.6 cm³. In the crystallization zone 14 of theautoclave, a seed crystal of gallium nitride having a thickness of 100μm and a surface area of 2.5 cm², obtained by the HVPE method, wasplaced. Then the autoclave was charged with 1.2 g of metallic gallium of6N purity and 2.2 g of metallic potassium of 4N purity. Subsequently,the autoclave 1 was filled with 15.9 g of ammonia (5N), closed, put intoa set of furnaces 4 and heated to a temperature of 200° C. After 3days—during which period metallic gallium was dissolved in thesupercritical solution. The temperature was increased to 450° C. whichresulted in a pressure of about 2.3 kbar. The next day, thecrystallization zone temperature was increased to 500° C. while thetemperature of the dissolution zone 13 was lowered to 370° C. and theautoclave 1 was kept in these conditions for the next 20 days (see FIG.11). As a result of this process, the partial dissolution of thematerial in the dissolution zone 13 and the growth of the galliumnitride on gallium nitride seeds in the crystallization zone 14 tookplace. The resulting crystal of gallium nitride having a total thicknessof 350 μm was obtained in the form of two-sided monocrystalline layers.

Example 9

[0104] An amount of 3.0 g of gallium nitride in the form of a sinteredgallium nitride pellet was put into the dissolution zone 13 ofhigh-pressure autoclave 1 having a volume of 35.6 cm³ (see FIG. 9). Inthe crystallization zone 14 of the autoclave, a seed crystal of galliumnitride obtained by the HVPE method and having a thickness of 120 μm anda surface area of 2.2 cm² was placed. Then the autoclave was chargedwith 2.3 g of metallic potassium of 4N purity. Subsequently, theautoclave 1 was filled with 15.9 g of ammonia (5N), closed, put into aset of furnaces 4 and heated to a temperature of 250° C. in order topartially dissolve the sintered GaN pellet and obtain a preliminarysaturation of a supercritical solution with gallium in the soluble form.After two days, the crystallization zone 14 temperature was increased to500° C. while the temperature of the dissolution zone 13 was lowered to420° C. and the autoclave 1 was kept in these conditions for the next 20days (see FIG. 12). As a result of this process, partial dissolution ofthe material in the dissolution zone 13 and the growth of galliumnitride on the gallium nitride seed took place in the crystallizationzone 14. A crystal of gallium nitride having a total thickness of 500 μmwas obtained in the form of two-sided monocrystalline layers.

Example 10

[0105] An amount of 0.5 g of gallium nitride plates having an averagethickness of about 120 μm, produced by the HVPE method, were put intothe dissolution zone 13 of high-pressure autoclave 1 having a volume of35.6 cm³. In the crystallization zone 14 of the autoclave, three seedcrystals of gallium nitride obtained by the HVPE method were placed. Theseed crystals had a thickness of about 120 μm and a total surface areaof 1.5 cm². Then the autoclave was charged with 0.41 g of metalliclithium of 3N purity. Subsequently, the autoclave 1 was filled with 14.4g of ammonia (5N), closed, put into a set of furnaces 4 and heated sothat the temperature of the crystallization zone 14 was increased to550° C. and the temperature of the dissolution zone 13 was increased to450° C. The resulting pressure was about 2.6 kbar. The autoclave 1 waskept in these conditions for the next 8 days (see FIG. 13). As a resultof this process, partial dissolution of the material in the dissolutionzone 13 and growth of gallium nitride on the gallium nitride seeds inthe crystallization zone 14 took place. The resulting crystals ofgallium nitride had a thickness of 40 μm and were in the form oftwo-sided monocrystalline layers.

Example 11

[0106] An amount of 0.5 g of gallium nitride having an average thicknessof about 120 μm, produced by the HVPE method, was placed into thedissolution zone 13 of high-pressure autoclave 1 having a volume of 35.6cm³. In the crystallization zone 14 of the autoclave, three seedcrystals of gallium nitride obtained by the HVPE method were placed. Theseed crystals had a thickness of 120 μm and a total surface area of 1.5cm². Then the autoclave was charged with 0.071 g of metallic gallium of6N purity and 1.4 g of metallic sodium of 3N purity. Subsequently, theautoclave 1 was filled with 14.5 g of ammonia (5N), closed, put into aset of furnaces 4 and heated to temperature of 200° C. After 1day—during which period metallic gallium was dissolved in thesupercritical solution—the autoclave 1 was heated so that thetemperature in the crystallization zone was increased to 500° C., whilethe temperature in the dissolution zone was increased to 400° C. Theresulting pressure was about 2.3 kbar. The autoclave 1 was kept in theseconditions for the next 8 days (see FIG. 14). As a result of thisprocess, partial dissolution of the material in the dissolution zone 13and growth of gallium nitride on gallium nitride seeds in thecrystallization zone 14 took place. The resulting crystals of galliumnitride were obtained in the form of two-sided monocrystalline layershaving a total thickness of 400 μm.

Example 12

[0107] An amount of 0.5 g of gallium nitride having an average thicknessof about 120 μm, produced by the HVPE method, was placed into thedissolution zone 13 of the high-pressure autoclave 1 having a volume of35.6 cm³. In the crystallization zone 14 of the autoclave, three seedcrystals of gallium nitride obtained by the HVPE method were placed. Theseed crystals had a thickness of 120 μm and a total surface area of 1.5cm². Then the autoclave was charged with 0.20 g of gallium amide and 1.4g of metallic sodium of 3N purity. Subsequently, the autoclave 1 wasfilled with 14.6 g of ammonia (5N), closed, put into a set of furnaces 4and heated to a temperature of 200° C. After 1 day during which periodgallium amide was dissolved in the supercritical solution—the autoclave1 was heated so that the temperature in the crystallization zone wasincreased to 500° C., while the temperature in the dissolution zone wasincreased to 400° C. The resulting pressure was about 2.3 kbar. Theautoclave 1 was kept in these conditions for the next 8 days (see alsoFIG. 14). As a result of this process, partial dissolution of thematerial in the dissolution zone 13 and growth of gallium nitride ongallium nitride seeds in the crystallization zone 14 took place. Theresulting crystals of gallium nitride were in the form of two-sidedmonocrystalline layers having a total thickness of 490 μm.

Example 13

[0108] One crucible was placed into the above-mentioned high-pressureautoclave having a volume of 10.9 cm³. The crucible contained 0.3 g ofthe feedstock in the form of metallic gallium of 6N purity. Also threegallium-nitride seeds having a thickness of about 0.5 mm and a totalmass of 0.2 g, all obtained by the HVPE method, were suspended withinthe reaction chamber. Further, 0.5 g of metallic sodium of 3N purity wasplaced in the autoclave; the autoclave was filled with 5.9 g of ammoniaand then closed. The autoclave was put into a furnace and heated to atemperature of 200° C., where the pressure was about 2.5 kbar. After 1day the temperature was increased to 500° C., while the pressureincreased up to 5 kbar and the autoclave was maintained in theseconditions for further 2 days. As a result of this process, thefeedstock was completely dissolved and the crystallization of galliumnitride took place on the seed. The average thickness of thetwo-side-overgrown monocrystalline layer of gallium nitride was about0.14 mm. The FWHM of the X-ray rocking curve from the (0002) plane atthe gallium-terminated side was 43 arcsec, while at thenitrogen-terminated side it was 927 arcsec.

[0109] The monocrystalline layers have a wurzite structure like in allof the other examples.

What is claimed is:
 1. A gallium-containing nitride crystal having asurface area of more than 2 cm² and having a dislocation density of lessthan 10⁶/cm².
 2. The gallium-containing nitride crystal according toclaim 1, wherein the gallium-containing nitride crystal has the generalformula Al_(x)Ga_(1−x−y)In_(y)N, where 0≦x<1, 0≦y<1, 0≦x+y<1.
 3. Thegallium-containing nitride crystal according to claim 1, wherein thegallium-containing nitride crystal contains alkali elements in an amountof more than about 0.1 ppm.
 4. The gallium-containing nitride crystalaccording to claim 1, wherein the gallium-containing nitride crystal hasa halogen content of about 0.1 ppm or less.
 5. The gallium-containingnitride crystal according to claim 1, wherein the gallium-containingnitride crystal has a volume of more than 0.05 cm³.
 6. Thegallium-containing nitride crystal according to claim 1, wherein thegallium-containing nitride crystal contains at least one elementselected from the group consisting of Ti, Fe, Co, Cr, and Ni.
 7. Thegallium-containing nitride crystal according to claim 1, wherein thegallium-containing nitride crystal additionally contains at least one ofa donor dopant, an acceptor dopant and a magnetic dopant, in aconcentration from 10¹⁷ to 10²¹/cm³.
 8. The gallium-containing nitridecrystal according to claim 1, wherein the layer of gallium-containingnitride crystal further contains at least one of Al and In and whereinthe molar ratio of Ga to the at least one of Al and In is more than 0.5.9. The gallium-containing nitride crystal according to claim 1, whereinthe gallium-containing nitride crystal contains a seed.
 10. Thegallium-containing nitride crystal according to claim 1, wherein thegallium-containing nitride crystal is monocrystalline.
 11. Agallium-containing nitride crystal having a thickness of at least 200 μmand a full width at half maximum (FWHM) of X-ray rocking curve from(0002) plane of 50 arcsec or less.
 12. The gallium-containing nitridecrystal according to claim 11 wherein the thickness is at least 500 μm.13. The gallium-containing nitride crystal according to claim 12,wherein the gallium-containing nitride crystal has the general formulaAl_(x)Ga_(1−x−y)In_(y)N, where 0≦x<1, 0≦y<1, 0≦x+y<1.
 14. Thegallium-containing nitride crystal according to claim 12, wherein thegallium-containing nitride crystal contains alkali elements in an amountof more than about 0.1 ppm.
 15. The gallium-containing nitride crystalaccording to claim 12, wherein the gallium-containing nitride crystalhas a halogen content of about 0.1 ppm or less.
 16. Thegallium-containing nitride crystal according to claim 12, wherein thegallium-containing nitride crystal has a volume of more than 0.05 cm³.17. The gallium-containing nitride crystal according to claim 12,wherein the gallium-containing nitride crystal contains at least oneelement selected from the group consisting of Ti, Fe, Co, Cr, and Ni.18. The gallium-containing nitride crystal according to claim 12,wherein the gallium-containing nitride crystal additionally contains atleast one of a donor dopant, an acceptor dopant and a magnetic dopant,in a concentration from 10¹⁷ to 10²¹/cm³.
 19. The gallium-containingnitride crystal according to claim 12, wherein the layer ofgallium-containing nitride crystal further contains at least one of Aland In and wherein the molar ratio of Ga to the at least one of Al andIn is more than 0.5.
 20. The gallium-containing nitride crystalaccording to claim 12, wherein the gallium-containing nitride crystalcontains a seed.
 21. The gallium-containing nitride crystal according toclaim 12, wherein the gallium-containing nitride crystal ismonocrystalline.
 22. The gallium-containing nitride crystal according toclaim 11, wherein the gallium-containing nitride crystal has the generalformula Al_(x)Ga_(1−x−y)In_(y)N, where 0≦x<1, 0≦y<1, 0≦x+y<1.
 23. Thegallium-containing nitride crystal according to any of claim 11, whereinthe gallium-containing nitride crystal contains alkali elements in anamount of more than about 0.1 ppm.
 24. The gallium-containing nitridecrystal according to claim 11, wherein the gallium-containing nitridecrystal has a halogen content of about 0.1 ppm or less.
 25. Thegallium-containing nitride crystal according to claim 11, wherein thegallium-containing nitride crystal has a volume of more than 0.05 cm³.26. The gallium-containing nitride crystal according to claim 11,wherein the gallium-containing nitride crystal contains at least oneelement selected from the group consisting of Ti, Fe, Co, Cr, and Ni.27. The gallium-containing nitride crystal according to claim 11,wherein the gallium-containing nitride crystal additionally contains atleast one of a donor dopant, an acceptor dopant and a magnetic dopant,in a concentration from 10¹⁷ to 10²¹/cm³.
 28. The gallium-containingnitride crystal according to claim 11, wherein the layer ofgallium-containing nitride crystal further contains at least one of Aland In and wherein the molar ratio of Ga to the at least one of Al andIn is more than 0.5.
 29. The gallium-containing nitride crystalaccording to claim 11, wherein the gallium-containing nitride crystalcontains a seed.
 30. The gallium-containing nitride crystal according toclaim 11, wherein the gallium-containing nitride crystal ismonocrystalline.
 31. Substrate for epitaxy crystallized on the surfaceof a crystallization seed, wherein the substrate includes a layer ofbulk monocrystalline gallium-containing nitride, having a surface areaof more than 2 cm² and a dislocation density of less than 10⁶/cm². 32.Substrate for epitaxy according to claim 31, wherein the layer of bulkmonocrystalline gallium-containing nitride has the general formulaAl_(x)Ga_(1−x−y)In_(y)N, where 0≦x<1, 0≦y<1, 0≦x+y<1.
 33. Substrate forepitaxy according to claim 31, wherein the substrate contains alkalielements in an amount of more than about 0.1 ppm.
 34. Substrate forepitaxy according to claim 31, wherein the layer of bulk monocrystallinegallium-containing nitride has a halogen content that does not exceedabout 0.1 ppm.
 35. Substrate for epitaxy according to claim 31, whereinthe layer of bulk monocrystalline gallium-containing nitride has volumeof more than 0.05 cm³.
 36. Substrate for epitaxy according to claim 31,wherein in the layer of bulk monocrystalline gallium-containing nitridehas a full width at half maximum (FWHM) of X-ray rocking curve from(0002) plane of less than 600 arcsec.
 37. Substrate for epitaxyaccording to claim 31, wherein the layer of bulk monocrystallinegallium-containing nitride additionally contains at least one of a donordopant, an acceptor dopant and a magnetic dopant, in a concentrationfrom 10¹⁷ to 10²¹/cm³.
 38. Substrate for epitaxy according to claim 31,wherein the layer of bulk monocrystalline gallium-containing nitridecontains at least one of Al and In and wherein the molar ratio of Ga tothe at least one of Al and In is more than 0.5.
 39. Substrate forepitaxy according to claim 31, wherein the layer of bulk monocrystallinegallium-containing nitride is crystallized on the surface of acrystallization seed of gallium-containing nitride having a dislocationdensity of less than 10⁶/cm².
 40. Substrate for epitaxy according toclaim 31, wherein the layer of bulk monocrystalline gallium-containingnitride has a dislocation density of less than 10⁴/cm² and a full widthat half maximum (FWHM) of X-ray rocking curve from (0002) plane of lessthan 60 arcsec.